In designing color image capture and reproduction systems, it is important to be able to render colors in an optimal manner such that they are most meaningful, appropriate, natural, and pleasing. Various modes of color reproduction are applicable in different color reproduction applications. For instance, a color image in a fine art publication may well be judged in terms of the accuracy of color appearance of the reproduction relative to the original artwork. Images intended for advertising may be judged in terms of the calorimetric accuracy of the reproduction of certain trademark colors against an aim. Color reproductions of analytical test charts such as the Macbeth™ color checker are judged relative to the spectral accuracy of the reproduction patches compared to the objects those patches represent. Pictorial images are generally judged against more arbitrary standards. Images used in commercial applications are often rendered more colorful, making them more appealing and attention grabbing. Professional portrait photographers will always print skin tones that please the subject rather than provide the most accurate rendition. Personal photographs, or snapshots, are frequently judged relative to a memory of how the original scene appeared. These memories are seldom colorimetrically accurate and are frequently influenced by color preferences of the customer, particularly for the so-called memory colors including sky, foliage and skin tones.
A comprehensive discussion of color rendition can be found in R. W. G. Hunt, The Reproduction of Colour, 5th Edition, Fountain Press, pp. 223-242 (1995). Hunt describes how color images can be reproduced in a range of different ways, to achieve a number of different objectives. In each case, however, observers judge the color quality of that reproduction according to their intended use of the image. Hunt has identified six distinct modes of color reproduction that cover all applications of color reproduction from entertainment to scientific:
1. The most demanding mode of color reproduction is “spectral reproduction”. This is where the spectral reflectance (or transmittance) of the reproduction matches that of the original. In this case the reproduction and the original will match in appearance regardless of changes in illuminant for any observer. Spectral color reproduction is seldom achieved in practice and is not commercially viable as a means of image reproduction.
2. In “colorimetric color reproduction” the chromaticities and relative luminances of the original and reproduction match for a defined viewing illuminant and a defined standard observer. Since color appearance is not independent of viewing illuminant intensity, calorimetric reproductions do not match originals in color appearance for large intensity changes.
3. “Exact color reproduction” occurs when the conditions for calorimetric color reproduction are met and, the absolute luminances of the reproduction and original match. The original and reproduction will match in appearance for a standard observer under identical conditions of visual system adaptation.
4. If a reproduction is to be viewed under different conditions of adaptation than the original then a color appearance match can only be achieved if adaptation factors are considered and accounted for in the mode of reproduction. This is the case with “equivalent color reproduction”. This is very difficult to achieve in practice because the colorfulness of objects seen under bright daylight usually cannot be reproduced under artificial illuminant viewing conditions.
5. In “corresponding color reproduction” the objective is to reproduce an original scene as it would appear if it were viewed under the reproduction viewing conditions.
6. The final mode of color reproduction is “preferred color reproduction”. In this mode the reproduction colors are rendered to match the preference of the observer. These preferences will tend to vary from observer to observer but there are general, quantifiable preferences that exist across most observers that can be included in a color reproduction.
In addition to the Hunt book, there are a number of publications in the technical literature that refer to the importance of preferred colors and concept of key memory colors in the art of rendering natural images. For example, C. J. Bartleson, “Memory Colors of Familiar Objects,” Journal of the Optical Society of America, 50, pp. 73-77, (1960); Siple et al., “Memory and Preference for the Colors of Objects,” Perception and Psychophysics, 34, pp. 363-370, (1983); S. Sanders, “Color Preferences for Natural Objects,” Illuminating Engineering, 54, p. 452, (1959); and Hunt et al., “The Preferred Reproduction of Blue Sky, Green Grass and Caucasian Skin in Colour Photography,” Journal of Photographic Science,” 22, pp. 144-150, (1974). It is evident from this literature that, for some objects whose colors are well-known, preferred color reproduction may be advantageous, wherein departures from equality of appearance (whether at equal or at different absolute luminance levels) may be required in order to achieve a more pleasing rendition.
As a consequence, imaging and photographic companies have intuitively understood the benefits of preferred color rendition for many years and have attempted to achieve some measure of customer preference through careful manipulation of the characteristics, including tone scale, interimage and spectral sensitivity, of the photographic system. In contemporary imaging systems, preferred color rendering is often accomplished through the use of some form of preferred color mapping. For example, U.S. Pat. No. 5,583,666 issued Dec. 10, 1996 to Ellson et al., describes a method for transforming an input color space to an output color space using a transform. In particular, a computer graphics morphing technique is described wherein the explicit constraints are comprised of points residing on the gamut boundary of the input and output color spaces. Although this method produces a smooth output rendition, it is limited in terms of the constraints it can impose on the color space transformation. An arbitrary point in the color space cannot be moved to another arbitrary position in color space. A group of patents issued to Buhr et al. (U.S. Pat. Nos. 5,528,330; 5,447,811; 5,390,036; and 5,300,381) describe color image reproduction of scenes with preferential tone mapping for optimal tone reproduction in color photographs. In this series of patents, the image reproduction is modified by a scene parameter transformation which, when taken in conjunction with untransformed characteristics of the image reproduction system and method, results in a reproduced tone mapping having instantaneous gamma values with a prescribed set of properties. This produces a reproduction having preferred visual characteristics with respect to tones, but does not address the issues of preferred color nor does it provide any method of generating a preferred color reproduction. Such tonal mappings, when applied to color data represented in an additive RGB type of color space, also result in chroma modifications. These chroma modifications are coupled to the tonal mapping and achieve chroma increases only as a consequence of luminance contrast increases. There is no provision in such methods for independent control of luminance and chrominance mappings. Furthermore tonal mappings such as these introduce uncontrolled hue shifts for most colors whenever the tonal mapping deviates from linearity. Another limitation of tonal mappings is that there is no ability to selectively apply the transform to limited regions of the color space.
As mentioned above, in contemporary imaging systems, preferred color rendering is often accomplished through the use of some form of preferred color mapping that is typically achieved through limited, slow and inflexible means. These means include, but are not limited to, modification of tonescales, modifications of selected colors, change of dyes or colorants, or manual manipulation of digital images using products such as Adobe Photoshop®. Because the perception of the light received by the eye and interpreted as a natural image is a very complex process, the subtle color and tonescale changes inherent in natural images are not well accommodated by simple transformations. As a result, a myriad of artifacts and unnatural appearing problems occur. These problems can include transitions between colors across gradients of hue, chroma, or lightness that are not as smooth or continuous as they appear in the natural scene. These transitions may produce artifacts or discontinuities in the image. In addition, the calculations necessary to compute the required transforms are often complex and may require dedicated computational resources and time. The time required for accomplishing the calculations often limits the applicability of the methods because they cannot be done in real time as images are physically produced and as a result must be done off line or else a substantial sacrifice in productivity will result.
What is needed is a completely new algorithm that enables colors to be transformed to produce a preferred color mapping, preferably by moving colors to or toward preferred positions in a color space. Ideally, when natural images are processed, gradations in color or between colors should appear as smooth or sharp in the modified images as they did in the original. Specifically, continuous transitions should be smooth and continuous and should not produce artifacts, discontinuities or contours. In addition, the overall impression of the composition of the image should be such that the rendered image is more pleasing than prior to treatment with the algorithm.
These transitions must also be able to be easily incorporated into systems in which the input color gamut differs from the output color gamut. Shu et al. (U.S. Pat. No. 6,400,843 B1) describe a method for deriving compensation transforms to map colors from an input color gamut to an output color gamut. The method necessitates gamut characterization of both the input and output systems in order to obtain higher-quality color reproduction. Shu et al. do not describe an approach for applying preferred color to, and modifying color gamut of, an input image independent of input and output gamut characterization.
Due to the plethora of combinations of input color systems and output color systems with varying color gamuts, a generic and robust means of applying preferred color transforms to an image is needed. There is a need therefore for an algorithm for moving image colors to or toward preferred positions that avoids the requirement of providing a full characterization of the input and output gamuts of the respective input and output color systems.